Method of producing an initial thermal arrest in the cooling curve of hypereutectic cast iron



Dec. 15, 1970 Filed Aug. 7, 1967 TIME w. T. BOURKE ETAL 3,546,921 METHOD OF PRODUCING AN INITIAL THERMAL ARREST IN THE COOLING CURVE OF HYPEREUTECTIC CAST IRON 2 Sheets-Sheet l l I I l I I I I l j 0 O O o O o O o O o O o In 0 LO 0 LO 0 LO 0 In 0 O O (\I (\I In N) 1 q- [0 cu (\l (\I N (\l cu m m (\l m m TEMPERATURE F I I I I I l I I I I O O O O O O O O O O O O n 0 In 0 In 0 In 0 In 0 O O m N m m wr LO cu (\I m N m cu cu cu cu (\I m TEMPERATURE F Dec. 15, 1970 w. 'r. BOURKE ET AL 3,546,921

METHOD OF PRODUCING AN INITIAL THERMAL ARREST IN THE COOLING CURVE OF HYPEREUTECTIC CAST IRON Filed Aug. '7. 1967 2 Sheets-Sheet 2 Fig.3

4.3 44 4.'5 4's 4.'? 4.8 4.9 PERCENT CARBON EQUIVALENT METHOD OF PRODUCING AN INITIAL THERMAL ARREST IN THE COOLING CURVE 0F HYPER- EUTECTIC CAST RON William T. Bourke, Suifern, N.Y., Spencer Harris, Kinuelou Borough, N1, and Tom C. Muff, Easton, Pa., assiguors to Harris-Muff, Inc., Kinnelon, N.J., a corporation of New Jersey Filed Aug. 7, 1967, Ser. No. 658,838 Int. Cl. GOIn 25/02 U.S. Cl. 73-17 12 Claims ABSTRACT OF THE DISCLOSURE Method of producing an initial thermal arrest in the cooling curve of a molten sample of hypereutectic cast iron by introducing into the sample a stabilizing additive having the characteristic of retarding primary graphite formation as the molten sample cools to its freezing tem perature.

FIELD OF THE INVENTION This invention relates to the monitoring of constituents of molten cast iron, and has for an object a method of determining the carbon equivalent of the iron as it existed in the molten state prior to processing and/or the me chanical and physical properties of the iron.

DESCRIPTION OF THE PRIOR ART The principle of carbon equivalent determination by pyrometry is based upon the accurate measurement of the initial or liquidus thermal arrest temperature which occurs in a sample of molten cast iron as it begins to freeze. The carbon equivalent (CE) may be defined a the total percent of carbon plus one-third of the total percent of silicon plus one-third of the total percent of phosphorus contained in a sample of cast iron, based upon the total weight of the sample. Under carefully controlled conditions, the liquidus temperature for hypoeutectic cast irons, i.e., cast iron having a carbon equivalent less than 4.35 percent, is easily observed. The heat liberated when austenite starts to precipitate produces an isothermal arrest of the cooling curve.

The temperature at which the liquidus thermal arrest occurs is related directly to the carbon equivalent of the metal and is not affected appreciably by normal amounts of manganese, chromium, nickel or other common contaminating elements. When accurately detected, the measurement of liquidus thermal arrest is consistently reproducible, and in actual foundry use is much faster and more reliable than chemical analysis. From a practical standpoint, the carbon and silicon contents are the two main variables in cast iron. as the phosphorus content is generally present in quantities low enough to be relatively ineffective for any given carbon equivalent. The silicon content can be estimated on the basis of chill tests, thus leaving only the carbon content to be determined. By determining the carbon equivalent from a cooling curve of the sample of cast iron, the carbon content can be calculated from its relationship to the carbon equivalent. With this knowledge of the carbon content, a foundry will be aware, before pouring, whether the composition of the cast iron meets the required specifications.

A suitable expendable phase change detector device for use in determining the carbon equivalent of molten cast United States Patent 0 3,546,921 Patented Dec. 15, 1970 "ice iron is disclosed in U.S. Pat. No. 3,267,732, reissued a U.S. Re. Pat. No. 26,409. The carbon equivalent technique involving the cooling curve test is also described in an article entitled Carbon Equivalent in Sixty Seconds, which appeared in the March 1962 issue of Modern Castings, at pages 37-39. In that article, at page 38, it is pointed out that the liquidus-break for hypereutectic irons, i.e., those having a carbon equivalent equal to or greater than about 4.35 percent, the eutectic point, is not clear and definite enough to be drawn with this test and, therefore, the test is limited to hypoeutectic iron, i.e., those having a carbon equivalent of less than 4.3 percent.

It has been suggested to extend the cooling curve test for carbon equivalent determination of hypereutectic cast irons by a technique involving the dilution of the hypereutectic iron sample with a known amount of a lowcarbon steel and subsequently determining the carbon equivalent as conventionally performed with cooling curves for hypoeutectic iron. After the carbon equivalent of the steel and cast iron mix is determined, a correction is added to determine the carbon equivalent of the cast iron sample. However, since the steel additive did not always melt and mix with the sample, the carbon equivalent is not reduced the full amount and the correction is inaccurate. While the use of low carbon steel wire in the form of coiled springs spatially distributed throughout the sample carrier has improved the reliability of the foregoing dilution technique, the care required in correctly spacing the fine wire in the container to obtain reliable results and in establishing the proper correction factor diminishes the attractiveness of this method.

The present invention enables the carbon equivalent technique to be extended to hypereutectic iron up to the kish point, i.e., the temperature at which free graphite forms and floats out of molten hypereutectic cast iron as it cools, without resort to any dilution technique which changes the carbon equivalent of the cast iron sample from that of the melt.

SUMMARY OF THE INVENTION Hypereutectic cast iron tends to product stable graphite when cooling from the melt to the freezing temperature of the cemenite through the formation of unstable iron carbide which immediately decomposes to iron and carbon (graphite). This causes complex heat effects resulting in a poorly defined arrest of the cooling curve. It has been discovered, however, that when the carbide is stabilized to retard complete graphite formation, simple freezing of iron carbide is achieved, causing an arrest of the cooling curve at the carbide forming temperature due to the higher heat of formation of iron carbide than of graphite.

In accordance with one form of the present invention, there is provided a method of introducing carbide stabilizer to a sample of the molten hypereutectic cast iron in an amount sufiicient to develop a discernible initial thermal arrest of the cooling curve of the sample at the carbide forming temperature and thereby permit determination of the carbon equivalent of the cast iron as it existed in the molten state prior to processing and/ or the determination of certain mechanical and physical properties of the iron which can be predicted from the initial arrest temperature.

Any material having the characteristic of retarding primary graphite formation during cooling of the molten sample of the hypereutectic cast iron to its freezing temperature, which is inert with respect to the carbon equivalent of the hypereutectic cast iron melt with which it is combined (in the sense that it does not change the carbon equivalent of the sample from that of the melt) may be used as a stabilizer in the practice of the invention. Generally, stabilizer characterized by a high degree of carbide stabilizing power are readily soluble in and dispersable throughout the molten sample of iron and will produce a consistent, discernible thermal arrest temperature when the iron carbide transformation occurs.

Stabilizers exhibiting the foregoing characteristics include bismuth, boron, cerium, lead, magnesium, and tellurium. Such tabilizers need not be combined with the molten hypereutectic cast iron sample in elemental form but may be added in compound form, or in mixtures with other materials which do not change the carbon equivalent of the cast iron sample from that of the melt. For example, boron may be added in the form of ferroboron (FeB). Cerium may be added in the form of misch metal (a mixture of rare earth elements, atomic numbers 57 through 71, in metallic form). Magnesium may be added in the form of copper-magnesium (Cu-Mg) with the magnesium being about percent by weight. Various combinations or mixtures of the foregoing additives have also been found successful in producing initial thermal arrest temperatures in cooling samples of hypereutectic cast iron. For example, the following mixtures have all provided useful initial thermal arrest temperatures: a mixture of tellurium, boron and mischmetal; a mixture of boron and mischmetal; a mixture of lead and mischmetal; and a mixture of bismuth and boron.

As will be appreciated by those skilled in the art, graphitizing materials, i.e., those promoting graphite formation during the Cooling of the hypereutectic cast iron sample should be avoided in the practice of the invention. For example, a material such as ferrosilicon (FeSi) is unsuitable since it not only promotes graphitization but also changes the carbon equivalent of the cast iron sample from that of the melt.

The amount of stabilizer used in the practice of the invention may vary within wide limits depending upon the particular stabilizer used, the carbon content of the sam ple, and the amount and kind of other constituents of the molten sample. Satisfactory curves may be obtained from cooling samples of cast iron containing as little as 0.05 weight percent (based upon the weight of the sample) of stabilizer. Preferably, the amount of stabilizer advantageously employed is the minimum amount required to obtain the desired retardation of primary graphite formation and accompanied arrest of the cooling curve at the carbide forming temperature. Although amounts of stabilizer as high a 0.4 percent have successfully been used, lesser amounts are generally preferred.

The addition of the stabilizers to molten cast iron samples in accordance with the invention may be accomplished in any manner so long as the temperature of the molten sample at the time of stabilizer addition is sufiiciently high to afford the production of the desired cooling curve. For example, the stabilizer may be added in pellet or particulate form to the molten sample immediately after after the sample is poured. Alternatively, the stabilizer can be added to the sampling device prior to the introduction of the molten sample. Other methods of combining the stabilizer with the sample will readily occur to those skilled in the art.

For further objects and advantages of the invention and for a more detailed description thereof, reference is to be had to the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cooling curve of a sample of hypereutectic cast iron produced in accordance with the present invention;

FIG. 2 is a cooling curve of a sample of hypereutectic cast iron having the same composition as the sample of FIG. 1 but poured in conventional manner without an additive; and

FIG. 3 is a graph showing the relationship of percent carbon equivalent of hypereutectic cast iron to initial thermal arrest temperature using carbide-stabilizing additives in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is illustrated a cooling curve of a sample of hypereutectic cast iron obtained in accordance with the present invention by combining with the sample while in liquid state a stabilizer having the characteristic of retarding primary graphite formation during cooling of the sample to the freezing temperature. The hypereutectic cast iron sample of FIG. 1 had a composition of 4.12 weight percent carbon, 1.62 weight percent silicon, and 0.076 weight percent phosphorus. This composition gave a carbon equivalent (CE), [CE=percent C-i- /a (percent Si+percent P)], of 4.69 percent.

The sample was poured into a phase-change detector device of the type disclosed in the aforesaid US. Pat. 3,267,732. Such detector device comprises a small cup having a volume of about ten cubic inches or less. A thermocouple of chromel-alumel wires extends through a wall thereof and is adapted to have its hot junction completely surrounded by the sample when poured into the cup and below any shrinkage cavity formed in the sample upon cooling. The thermocouple is adapted to be connected to a suitable chart recorder which traces the cooling curve of the sample as its temperature cools. The electrical connection between the thermocouple of the detector and the temperature measuring circuit of the recorder is completed by plugging the contacts of the detector into he mating contacts of a stand which is adapted to support the detector in vertical position for receiving the sample.

In obtaining the cooling cunve of FIG. 1, cerium, in the form of a one-gram pellet of mischmetal, was in serted in the cup prior to pouring the molten sample into the cup. The sample had a Weight of about 500 grams. As above noted, the amount of stabilizer may vary throughout a wide range. For example, satisfactory curves have been produced by the addition of as little as one-fourth gram of mischmetal, and as much as two grams, to a SOD-gram molten sample of hypereutectic cast iron including 4.2 percent C, and 1.5 percent Si.

Referring to FIG. 1, it will be seen that when the molten sample of hypereutectic cast iron was poured into the cup, the pen of the recorder moved up-scale from point A to point B until the actual temperature at the center of the sample was reached. At this point the recorder started to chart the cooling pattern. The liquidus or initial thermal arrest temperature appeared as a vertical segment as indicated at C on the curve, such temperature being 2200 F. The initial thermal arrest temperature normally is reached in about 20 to 40 seconds, depending on superheat, and this temperature can then be converted to carbon equivalent by using predetermined data of percent carbon equivalent versus temperature. A chart based on such predetermined data is illustrated in FIG. 3. From this chart, it will be noted that a sample of hypereutectic cast iron having an initial thermal arrest temperature of 2200 'F. has a carbon equivalent of 4.7 percent. This compares closely with the carbon equivalent of 4.69 percent by chemical analysis of the sample.

A photomicrograph of the microstructure of a picrol etched sample of hypereutectic cast iron having the cooling curve illustrated in FIG. 1 showed the existence of carbon (graphite) in relatively small amounts.

Referring to FIG. 2, there is illustrated a cooling curve of a hypereutectic cast iron sample having the same composition and poured from the same ladle as the sample of FIG. 1 but poured into the cup of a carbon equivalent detector without the addition of a stabilizer. This sample was poured concurrently with the sample of FIG. 1 but the cooling curve obtained from the sample not containing the stabilizer, as shown in FIG. 2, did not show a useful thennal arrest above the eutectic temperature arrest. Thus, it was not possible to determine the carbon equivalent from the cooling curve of FIG. 2 even though that sample was of the same iron and therefore had the same carbon equivalent and the same composition as the sample used to produce the cooling curve of FIG. 1.

A photomicrograph of the microstructure of a picrol etched sample of hypereutectic cast iron which produced the cooling curve of FIG. 2 showed worm-like graphite flakes long and massive as compared to short and fine graphite flakes of the etched sample which produced the curve of FIG. 1, thus indicating the effect of the mischmetal additive in inhibiting or retarding the formation and/or growth of graphite during cooling of the sample to the freezing temperature.

The initial thermal arrest in the cooling curve of a sample of hypereutectic cast iron, as indicated at C in FIG. 1, may be produced with various stabilizing additives having the characteristic of retarding primary graphite formation during cooling of the sample to the freezing temperature. Various stabilizers have been used to produce thermal arrest temperatures in samples of hypereutectic cast iron having percent carbon equivalent ranging from the eutectic point at 4.35 percent up to about 4.95 percent. The percent carbon equivalents for these various samples were determined from chemical analysis. By plotting the thermal arrest temperatures versus percent carbon equivalent of the samples, the curve of FIG. 3 was produced.

The following table of data shows the carbon equivalent of hypereutectic cast iron as determined from the correlation data of FIG. 3 and using carbide-stabilizing additives as compared to the percent carbon equivalent determined by chemical analysis.

Percent carbon equivalent Thermal tarrest F h 133; emp., mm c emlca Stabilizer addition F. correlation analysis Tellurium plus boron plus mischmet 2, 315 4. 84 4. 79 Boron plus mischmetaL 2, 323 4. 85 4. 85 Lead plus Inischm a1 2, 280 4. 8O 4. 80 2, 210 4. 71 4. 70 Miscgmet 2, 7(1) Boron plus mischmetaL 2, 217 4. 72 4. 65 Mischmetal 2, 192 4. 69 4. 51 2, 123 4. 58 4. 51 Bismut 2, 360 4. 89 4. 77 1:\B/I1S(.h111et 2, 343 4. 86 80 Mom. 2, 2 4.85 .68 Mischmetal. 2, 341 4. 87 4. 68 Bismuth plus boron 2, 310 4. 83 4. 68

It will be noted that the comparison of the percent carbon equivalent as determined from the initial thermal arrest temperature produced in the hypereutectic cast iron in accordance with the present invention is well within the useful limits. In fact, it has been found that this method of determining the percent carbon equivalent is more reliable than chemical analysis, as frequently chemical analyses of the same sample of cast iron will vary from one laboratory to another.

While the expendable phase-change detector device of the type disclosed in the aforesaid Pat. 3,267,732 and Re. 26,409 has been referred to specifically in describing the present invention, it is to be understood that any suitable phase-change detector device may be used. Such detector devices will normally comprise a cup member open at the top for receiving the molten sample of cast iron and having a temperature-responsive means extending into the cup for disposition below the surface of the sample so that it may sense the temperature change of the sample as it cools. The temperature sensing means may be of any suitable type, but will preferably be in the form of a thermocouple comprised of suitable materials depending upon the temperatures to be encountered. Other suitable carbon equivalent detector devices are disclosed in US. Pat. No. 3,321,973, British Pat. No. 944,302 and the February 1962 article from Modern Castings entitled Gray Cast Iron Control By Cooling Curve Techniques, pages 91 to 98.

While a preferred form of the invention has been described, it is to be understood that other modifications thereof may be made without departing from the spirit and scope of the appended claims.

What is claimed is:

1. A method of producing an initial thermal arrest in the cooling curve of a possibly hypereutectic cast iron, said curve being obtained by substantially continuously measuring and plotting a curve of the temperature while a sample of said iron is cooling from a molten to a solid state which comprises, adding to the sample of molten cast iron prior to any substantial cooling thereof a material inert with respect to the carbon equivalent thereof, in the sense that it does not change the carbon equivalent of the sample from that of the melt, which material retards primary graphite formation during cooling of said molten sample of said iron to its freezing temperature to insure an arrest of the cooling curve of said sample at the carbide forming temperature.

2. The method of claim 1 in which the added material includes at least one member of the group consisting of bismuth, boron, cerium, lead, magnesium, and tellurium.

3. The method of claim 1 in which the added material includes bismuth.

4. The method of claim 1 in which the added material includes boron.

5. The method of claim 1 in which the added material includes lead.

6. The method of claim 1 in which the added material includes magnesium.

7. The method of claim 1 in which the added material includes tellurium.

8. The method of claim 1 in which the added materialincludes cerium.

9. The method of claim 1 in which the added material comprises mischmetal.

10. The method of claim 1 wherein said step of adding to the sample a material inert with respect to the carbon equivalent of said molten cast iron is effected by including a quantity of said material on the inner surfaces of an open ended mold in which said molten sample is to be cast so that addition takes place when said sample is poured into said mold.

11. A method for producing an initial thermal arrest in the cooling curve of a sample of molten hypereutectic cast iron comprising sampling the melt, and

adding a stabilizer to the molten sample which retards primary graphite formation during cooling of the molten sample of said iron to its freezing temperature to cause an arrest of the cooling curve of said sample at the carbide-forming temperature While maintaining the carbon equivalent of the sample the same as the carbon equivalent of the melt.

12. A method of determining the carbon equivalent of molten cast iron which consists in determining the temperature of the thermal arrest point on the cooling curve for a sample of said molten cast iron which corresponds to the liquidus temperature wherein identification of the arrest point is facilitated by the addition to said sample of said molten cast iron of a material inert with respect to the carbon equivalent of said molten cast iron and which functions as a carbide stabilizer.

(References on following page) 8 v References Cited Jelley et a1.: How To Estimate Cupola Metal Composition Means Of Cooling Curves in Journal, 1961, vol. 9, pp. 622625. 3,375,106 3/1968 McKlsslck et a1. 75-130 3,415,307 12/1968 Schuh et a1 75 130 JAMES GILL, Primary Examiner 5 OTHER REFERENCES H. GOLDSTEIN, Assistant Examiner Clark et a1.: Determination of Boron Contents Greater Than 0.1 Percent in Cast Iron in BCIRA Journal, v01. 

